Measurement of the properties of materials is traditionally carried out using conventional laboratory analysis of a manually collected sample. This process is expensive and time consuming, and results for different material batches may be easily confused due to the time taken between sampling and the receiving of results. A device capable of delivering immediate results to the operator whilst in contact with the sample removes a large portion of potential errors and allows immediate decision making regarding the quality of said material due to its properties.
When a microwave signal passes through the material, some of the signal is absorbed such that the amplitude (i.e. power level) of the received microwave signal is less than was transmitted. The amount of attenuation may be directly related to the material property that is desired to be measured. In addition to monitoring attenuation, the velocity of the microwave signal is also affected by its passage through the material. This slowing of microwave velocity creates a phase shift in microwave signal. A phase shift is time difference between when a signal is received compared to when it should have been received with no interference. Attenuation and phase shift may be used to infer a number of compositional properties about the material being analysed.
GB 2,122,741 describes an apparatus for monitoring crushed coal. The apparatus monitors ash content and moisture content of the coal by respectively transmitting and detecting X-ray and microwave radiation. The microwave radiation amplitude is chopped at a low frequency of about 1.0 kHz, which is suitable for analysing a crushed sample such as coal. This apparatus, however, is not well suited for determining moisture content of other types of samples and is not easily portable.
U.S. Pat. No. 4,788,853 describes a moisture meter that also uses microwave signals at discrete discontinuous frequencies. This patent states that the number of frequencies required to perform the invention is not critical as long as sufficient data is generated. The device is designed for use with material moving on a conveyor belt.
AU 61689/90 describes an apparatus for determining moisture content in a sample of varying thickness on a conveyor belt. The microwave signals are also transmitted at discrete discontinuous frequencies within a selected range.
Although the above described apparatus may be useful for determining moisture content in a sample on a conveyor belt, these apparatus are nevertheless still prone to substantial errors or inaccuracies due at least in part to variation in sample depth or configuration. Furthermore the devices are not portable and therefore have limited usefulness.
In this specification, the terms “comprises”, “comprising” or similar terms are intended to mean a non-exclusive inclusion, such that a method, system or apparatus that comprises a list of elements does not include those elements solely, but may well include other elements not listed.